A variety of applications use polyphase direct current (dc) motors for providing rotational motion. In particular, applications such as hard disk drives and CD-ROM drives often use polyphase dc motors, such as three-phase dc motors, to rotate information platters including the magnetic disks of a hard disk drive. The control of the rotational speed of these information platters is often critical to overall application performance.
The rotational speed of these polyphase dc motors is controlled through the current applied to the stator windings or coils. For example, the stator windings of a three-phase dc motor may be coupled in a "Y" configuration and include an A-coil, a B-coil, and a C-coil coupled at one end at a center tap node. The remaining ends of each coil are selectively coupled to either a high side driver, a low side driver, or to an open circuit as commutation occurs. The drivers may be power transistors that are implemented as, for example, field-effect transistors (FET).
During steady state operation, current flows from a supply source coupled to the high side driver, through the high side driver, through a first coil coupled to the high side driver, through the center tap, through a second coil coupled to the low side driver, and through the low side driver which is also coupled to ground. During this time, a third coil couples to the center tap on one end and to an open circuit at the other end so that current does not flow through the third coil. After a period of time, a commutation occurs so that current may now flow through the third coil and either the first coil or the second coil depending on the commutation sequence. A commutation is the transfer of current from one path in a circuit to another. Thus, current flows through two of the three coils during a steady state operation until a commutation occurs, at which time, current then flows through one of the two coils and the third coil until the next commutation occurs.
A total of six currents may be provided in the stator windings of a three-phase dc motor through six commutations. The currents, for example, may be provided through the stator coils in the following current flow path sequence to impart rotational motion to the rotor of the three-phase dc motor: A-coil to C-coil, A-coil to B-coil, C-coil to B-coil, C-coil to A-coil, B-coil to A-coil, and B-coil to C-coil.
The current flows are controlled by the high and low side drivers provided at each phase of the stator windings. A commutation circuit sequentially applies control voltages to the gates of the respective high and low side drivers to cause the current to flow in the stator windings in a predetermined sequence such as is described above.
The currents in the respective current flow paths are desirably equal. However, due to variations in fabrication processes, mask alignments, and so on, the FET driver transistors, in the respective current flow paths, typically have variances and tolerances in such dimensions as the channel widths or lengths. These variances and tolerances result in inequalities in the currents in the respective current flow paths in the stator windings. As a result of these inequalities, audible noises, torque ripple, motor inaccuracies, and other undesirable effects may occur. Such unequal currents also reduce the precision of the motor speed. As data densities of hard disk drives become greater and greater, such imprecisions result in limitations on the data densities that might be achieved in a particular hard disk drive.
The stator winding currents may be detected and controlled using a control circuitry at each phase of the stator windings. This is generally performed at the low side driver transistors at each phase of the stator windings. This control circuity may include transistor FET devices connected as current mirrors to mirror the current in the FET driver transistors. Problems arise when the ratio of the size of the driver transistor to the mirror transistors is too small. If the ratio of the size of the driver transistor to the mirror transistors is too small, a large amount of power is wasted in the control circuitry because of the large current in the mirror transistors.
However, other problems are created when the ratio becomes too large. For example, a ratio on the order of 1500:1 reduces current mirror accuracy because of variances and tolerances that exist when fabricating mirror transistors of this size. The ratio can be expected to vary on a lot and device basis. Additionally, when the ratio becomes too large, these fabrication variances and tolerances introduce differences between the mirror transistors used in the control circuitry at each phase of the stator windings. This may ultimately contribute to the unequal current flows in the stator windings resulting in the problems mentioned above of audible noises, torque ripple, motor inaccuracies, and other undesirable effects. The audible noise may be in the 2-4 kHz range, which is a typical commutation frequency.
Furthermore, problems may also arise when the current flows in the stator windings are equivalent but at an undesirable amplitude. For example, if the current flows are too low, the motor may not provide the desired torque or rotational velocity.